1. Field of the Invention
The present invention relates to a hydraulic brake booster for operating wheel brakes with a magnified or boosted hydraulic pressure, and more particularly to an improvement of a hydraulic brake booster comprising a primary master cylinder for outputting a hydraulic pressure corresponding to an input from a brake pedal for the operation thereof, a hydraulic pressure source, a proportional pressure booster valve for drawing from the hydraulic pressure source a boost pressure corresponding to the hydraulic pressure outputted from the primary master cylinder for outputting and a secondary master cylinder for operating wheel brakes with a hydraulic brake pressure corresponding to the boost pressure outputted from the proportional pressure booster valve.
2. Description of the Related Art
The applicant has proposed before a hydraulic pressure booster described in JP-A-2000-177572, in which a primary master cylinder, a proportional pressure booster valve and a secondary master cylinder can be laid out individually and freely without interfering with one another, this making the hydraulic pressure booster advantageous in that the booster can easily be installed even in a limited narrow space.
In a case, however, where a tandem master cylinder is used for the primary master cylinder of the above hydraulic pressure booster, pairs of proportional pressure booster valves and secondary master cylinders are provided, respectively, in such a manner as to correspond to a pair of output ports provided in the primary master cylinder, this increasing the number of components involved in the booster, thereby making it difficult to reduce the production cost. In addition, scattering of sliding resistance of respective operating members of the pair of proportional pressure booster valves may cause a slight variation in timing at which the pair of secondary master cylinders are activated.
In addition, it is desirable for the proportional booster valve to output boost hydraulic pressure in accordance with pressure characteristics in which the pressure is drastically enhanced as if it were to jump in an initial operating stage of the primary master cylinder based on a viewpoint of eliminating a play portion from a hydraulic system extending from the proportional booster valve to the wheel brakes via the secondary master cylinders and thereafter is enhanced relatively moderately, and in order to obtain such pressure characteristics, it is considered to use as a reaction member the elastic material such as rubber used in the hydraulic booster which is disclosed in JP-A-11-20663. With the elastic material, however, since the elastic properties thereof vary depending upon temperatures, it is difficult with elastic material to obtain the aforesaid pressure characteristics in a stable fashion.
The invention was made in view of these situations, and an object thereof is to provide a hydraulic brake booster in which in addition to achieving the reduction in the number of components involved therein, a pair of secondary master cylinders can be made at all times to output hydraulic brake pressure simultaneously.
Further, another object of the invention is to provide a hydraulic brake booster which can stably obtain the hydraulic pressure characteristics in which the output from the proportional booster valve is jumped in an initial operating stage of the primary master cylinder.
With a view to attaining the object of the invention, according to a first aspect thereof, there is provided a hydraulic brake booster comprising a primary tandem master cylinder having a pair of output ports for outputting hydraulic pressure to an input to a brake pedal for the operation thereof, a hydraulic pressure source for outputting a hydraulic pressure which is higher than the hydraulic pressure outputted from the primary master cylinder, a plurality of secondary master cylinders each having an input pressure chamber and an output pressure chamber for outputting hydraulic pressure corresponding to the hydraulic pressure outputted from the input pressure chamber which is connected to wheel brakes, and a proportional pressure booster valve having a control piston facing at one end thereof a primary control pressure chamber to which one of both the output ports is connected, a free piston facing at both ends thereof a secondary control pressure chamber to which the other of both the output ports is connected and the primary control pressure chamber, respectively, and adapted to apply a direct pressing force to the control piston at the one end as the hydraulic pressure of the primary control pressure chamber decreases below the hydraulic pressure of the secondary control pressure chamber, a reaction unit for applying to the other end of the control piston a reaction force corresponding to the hydraulic pressure of a boost pressure chamber connected in common to the plurality of input pressure chambers, and a valve unit adapted to be activated as the control piston moves in an axial direction to there by draw from the hydraulic pressure source a hydraulic pressure proportional to the hydraulic pressure outputted from the primary master cylinder for introduction into the boost pressure chamber.
According to this construction, the hydraulic pressure outputted from both the output ports in the primary master cylinder which is operated by the input to the brake pedal to operate the same is applied to the ends of the free piston in the proportional pressure booster valve, and the free piston then operates such that the hydraulic pressure in the primary control pressure chamber between the control piston and the free piston balances with the hydraulic pressure in the secondary control pressure chamber which is located on the opposite side of the primary control pressure chamber and toward which the free piston faces at the one end thereof. Then, the proportional pressure booster valve outputs the boost pressure which is proportional to the hydraulic pressures of the primary and secondary control pressure chambers which balance with each other, and this hydraulic pressure so outputted from the proportional pressure booster valve then operates the plurality of secondary master cylinders, whereby the wheel brakes are applied strongly. Namely, since the plurality of secondary master cylinders are operated by virtue of the boost pressure outputted from the single common proportional pressure booster valve, the respective secondary master cylinders are always made to operate simultaneously with the boosted force without being affected by the sliding resistance that would occur on the operating members within the proportional pressure booster valve, whereby all the wheel brakes are applied simultaneously. According to this construction, the single proportional pressure booster valve suffices for the simultaneous operation of the plurality of secondary master cylinders and hence the number of components involved in the booster can be reduced, whereby the construction of the booster can be simplified, this contributing to the reduction in the production cost thereof. Moreover, even if there occurs a failure in the hydraulic system communicating with the primary control pressure chamber for some reason, the control piston can be pushed to operate directly with the hydraulic pressure in the secondary control pressure chamber, while there occurs a failure in the hydraulic system communicating with the secondary control pressure chamber for other reason, as long as the hydraulic pressure in the primary control pressure chamber remains normal, the control piston still can be pushed therewith, whereby the pressure boosting function of the proportional pressure booster valve can be maintained.
According to a second aspect of the invention, there is provided a hydraulic brake booster as set forth in the first aspect of the invention, wherein the output port of both the output ports of the primary master cylinder which is adapted to output at an earlier timing is connected to the primary control pressure chamber, and according to this construction, the hydraulic pressure outputted from the primary master cylinder is made to act on the control piston of the proportional pressure booster valve quickly, thereby making it possible to improve the response of the proportional pressure booster valve to the braking operation.
According to a third aspect of the invention, there is provided a hydraulic brake booster as set forth in the first or second aspect of the invention, wherein a small-diameter piston portion integrally provided at the one end of the control piston relatively slidably fits in the free piston which is a cylinder constructed so as to be bottomed at one end and made to open at the other end thereof which faces the control piston, and according to this construction, the control piston and the free piston are made to slide reciprocally, whereby the respective pistons can be movably supported so as to guarantee axial movements thereof with the axial length of the respective pistons being set short, thereby making it possible to contribute to the miniaturization of the proportional pressure booster valve.
According to a fourth aspect of the invention, there is provided a hydraulic brake booster as set forth in any of the first to third aspects of the invention, wherein a pair of annular seal members are mounted on an outer circumference of the free piston in such a manner as to be brought into a sliding contact with a housing in which the free piston slidably fits, and wherein a drain passage is formed in the housing for opening to the atmospheric pressure at a portion between the seal members, and according to this construction, not only can the driver of the vehicle be let known that there is occurring something abnormal in the seal on the outer circumference of the free piston, but also a risk attributable to the abnormality can be avoided in which the whole hydraulic brake circuit is reduced to a single hydraulic system. Namely, in the event that one of the two annular seal members fails for some reason, the hydraulic pressure in the hydraulic system connecting to the control pressure chamber associated with the annular seal member so failing is released to the atmospheric pressure, this increasing the operation stroke in the primary master cylinder, whereby the driver can be made to recognize that a certain failure is happening in the hydraulic brake systems. In addition, despite the drop in hydraulic pressure in one of the two pressure control chambers, the control piston is still able to be pushed on, and therefore, the boosting function can still be provided. In contrast to this, in a case where an only one annular seal member is mounted around the outer circumference of the free piston, in the event that the single annular seal member gets abnormal, the whole hydraulic brake circuit is reduced to a single hydraulic system, and should a hydraulic failure occur in either of the hydraulic systems, no counter measures can be taken.
According to a fifth aspect of the invention, there is provided a hydraulic brake booster comprising: a primary master cylinder for outputting hydraulic pressure in response to an input applied to a brake pedal for the operation thereof; a hydraulic pressure source for outputting a hydraulic pressure which is higher than the hydraulic pressure outputted from the primary master cylinder; secondary master cylinders each having an input pressure chamber and an output pressure chamber connected to wheel brakes for outputting hydraulic pressure corresponding to the hydraulic pressure of the input pressure chamber; and a proportional booster valve having a control piston facing at one end thereof toward a control pressure chamber into which the hydraulic pressure outputted from the primary master cylinder is inputted, a primary reaction piston for transmitting at all times to the control piston a reaction force corresponding to hydraulic pressure of a boost pressure chamber connected to the input pressure chamber, a secondary reaction piston for transmitting to the control piston a reaction force corresponding to the hydraulic pressure of the boost pressure chamber as the hydraulic pressure of the boost pressure chamber exceeds a set hydraulic pressure, and valve unit adapted to be activated as the control piston moves in an axial direction so as to draw from the hydraulic pressure source a hydraulic pressure proportional to the hydraulic pressure of the control pressure chamber for introduction into the boost pressure chamber.
According to this construction, the proportional booster valve output the boost pressure proportional to the hydraulic pressure outputted from the output ports of the primary master cylinder adapted to be activated by an input applied to the brake pedal to operate the same, and the secondary master cylinders are activated by virtue of the hydraulic pressure outputted from the proportional booster valve, where by the wheel brakes can be activated strongly, the brakes being then applied. In addition, in the proportional booster valve, in the initial operating stage of the primary master cylinder, the hydraulic pressure of the boost pressure chamber is applied to the control piston as a reaction force by only the primary reaction piston, and once the hydraulic pressure of the boost pressure chamber exceeds the set hydraulic pressure, the hydraulic pressure of the boost pressure chamber is applied to the control piston as the reaction force by both the primary and secondary reaction pistons, and therefore, the output from the proportional booster valve can be made to jump in the initial operating stage of the primary master cylinder, and moreover, the variation in the pressure characteristics due to temperatures can be avoided to thereby obtain stable pressure characteristics at all times.
According to a sixth aspect of the invention, there is provided a hydraulic brake booster as set forth in the fifth aspect of the invention, wherein the primary reaction piston fits in the secondary reaction piston fluid tightly and relatively slidably, the primary reaction piston being connected to the control piston at one end thereof and facing at the other end thereof toward the boost pressure chamber, the secondary reaction piston having formed at one end thereof an abutment face adapted to be brought into abutment with the primary reaction piston in such a manner as to face the primary reaction piston. According to this construction, despite the primary and secondary reaction pistons being disposed, the increase in the length of the proportional booster valve along an axial direction of the reaction pistons can be suppressed.
According to a seventh aspect of the invention, there is provided a hydraulic brake booster comprising: a primary master cylinder for outputting hydraulic pressure in response to an input applied to a brake pedal for the operation thereof; a hydraulic pressure source connected to a reservoir for outputting a hydraulic pressure higher than a hydraulic pressure outputted from the primary master cylinder irrespective of the operation of the brake pedal; secondary master cylinders each having an input pressure chamber and an output pressure chamber for outputting a hydraulic pressure corresponding to the hydraulic pressure of the input pressure chamber; a pressure regulator valve unit each provided between the output pressure chamber and a wheel brake for regulating the hydraulic pressure of the output pressure chamber; proportional pressure booster valves each having a control piston receiving at one end thereof a hydraulic pressure of a control pressure chamber into which the hydraulic pressure outputted from the primary master cylinder (M1) is inputted, a reaction unit for applying to the other end of the control piston a reaction force corresponding to a hydraulic pressure of a boost pressure chamber connected to the input pressure chamber, an inlet valve interposed between the boost pressure chamber and the hydraulic pressure source such that the inlet valve opens as the control piston moves toward a side where the capacity of the boost pressure chamber is reduced in response to a change in hydraulic pressure of the control pressure chamber and an outlet valve interposed between the boost pressure chamber and the reservoir such that the outlet valve closes as the control piston moves toward the side where the capacity of the boost pressure chamber is reduced for drawing a hydraulic pressure proportional to the hydraulic pressure of the control pressure chamber from the boost pressure chamber; a primary closing valve interposed between the hydraulic pressure source and the input pressure chamber; and a secondary closing valve provided at an intermediate position along a path extending from the input pressure chamber as far as the reservoir via the booster pressure chamber and the outlet valve such that the secondary closing valve opens when the primary closing valve closes, while the secondary closing valve closes when the primary closing valve opens.
According to this construction, when the primary master cylinder is activated by the driver of a vehicle who operates the brake pedal, the proportional booster valves output hydraulic pressure proportional to the hydraulic pressure outputted by the primary master cylinder. The hydraulic pressure so outputted from the proportional booster valves then activates the secondary master cylinders, whereby the wheel brakes are applied strongly. Moreover, the brake pressure at the wheel brakes can be controlled by regulating hydraulic pressure with the pressure regulator valve unit, so that the brake pressure control as in a case where the anti-lock braking system is activated on braking can be attained. In addition, the hydraulic pressure outputted from the hydraulic pressure source can be introduced into the input pressure chambers of the secondary master cylinders by opening the primary closing valves, whereby a braking assisting control and an emergency braking control can be effected quickly and properly. Moreover, the exertion of an excessive reaction force from the reaction unit to the control pistons is prevented which occurs as a high hydraulic pressure is introduced from the input pressure chambers of the secondary master cylinders into the boost pressure chambers of the proportional booster valves by closing the secondary closing valves while the primary closing valves are open. Additionally, even with the outlet valves being left open, the escape of the hydraulic pressure outputted from the hydraulic pressure source to the reservoir via the boost pressure chambers and outlet valves can be prevented.
In addition, according to an eighth aspect of the invention, there is provided a hydraulic brake booster as set forth in the first aspect of the invention, wherein the secondary closing valves are interposed between the boost pressure chambers and the input pressure chambers. According to this construction, a differential pressure between the hydraulic pressure in the hydraulic pressure source and the hydraulic pressure in the proportional booster valves is applied to valve bodies of the secondary closing valves in a valve opening direction when the secondary closing valves are open, and this helps reduce a force required to close the secondary closing valve, contributing to the miniaturization of the secondary closing valves.
According to a ninth aspect of the invention, there is provided a hydraulic brake booster as set forth in the seventh aspect of the invention, wherein a single number of the primary and secondary closing valves is provided, respectively, for the plurality of secondary master cylinders and the single hydraulic pressure source. According to this construction, even if there are provided the plurality of secondary master cylinders, only the single number of the primary and secondary closing valves may be required, this preventing the number of components from being increased.
According to a tenth aspect of the invention, there is provided a hydraulic brake booster as set forth in any of the seventh to ninth aspects of the invention, wherein the primary and secondary closing valves can be closed independently of the operation of the primary master cylinder. According to this construction, the secondary master cylinders can be activated independently of the operation of the primary master cylinder, whereby a stability control during a turn and automatic braking for keeping a proper distance between two vehicles running in succession can be performed.
According to an eleventh aspect of the invention, there is provided a hydraulic brake booster as set forth in the tenth aspect of the invention, including a detecting unit for detecting the operation of the primary master cylinder and a control unit (16) for controlling the primary and secondary closing valves such that the primary closing valve closes, while the secondary closing valve closes when the detecting unit detects the operation of the primary master cylinder in a state in which the primary closing valve are open, while the secondary closing valve are closed. According to this construction, when the driver performs a braking operation which activates the primary master cylinder while automatic braking in operation, a state can be restored in which the hydraulic pressure outputted from the proportional booster valves is applied to the secondary master cylinders.